Those skilled in the art know that the efficiency loss of a high pressure turbine increases rapidly as the blade tip-to-shroud clearance is increased, either as a result of blade tip wear resulting from contact with the turbine shroud or by design to avoid blade tip wear and abrading of the shroud. Any high pressure air that passes between the turbine blade tips and the turbine shroud does not do work and therefore is a system loss. Another loss is the use of compressor bleed air to cool the turbine shroud. If an insulative shroud technology could be provided which allows blade tip clearances to be small over the life of the turbine, there would be an increase in the overall turbine performance, including higher power output at a lower operating temperatures, better utilization of fuel, longer operating life, and reduced shroud cooling requirements.
To this end, efforts have been made in the gas turbine industry to develop abradable turbine shrouds to reduce clearance and associated leakage losses between the blade tips and the turbine shroud. Various techniques have been developed for coating turbine shrouds with ceramic materials such as, primarily, yttria stabilized zirconia. A disadvantage of these techniques is that the ceramic coating tends to spall off due to the steep thermal gradient across the thickness of the ceramic during engine operation. The spalling off severely reduces the sealing effectiveness and the insulative characteristics of the ceramic coating, causing shroud distortion, which results in a variation in the blade tip-to-shroud clearance, loss of performance, and expensive repairs.
Strangman, U.S. Pat. No. 4,914,794, entitled "Method of Making an Abradable Strain-Tolerant Ceramic coated Turbine Shroud", which is assigned to the assignee of this application and incorporated by reference herein, provides a solution to the spalling off problem. Strangman discloses an abradable ceramic coated turbine shroud structure which includes a grid of slant-steps isolated by grooves in a superalloy metal shroud substrate. A thin bonding layer is applied to the slant-steps, followed by a stabilized zirconia layer that is plasma sprayed at a sufficiently large spray angle to cause formation of deep shadow gaps in the zirconia layer. The shadow gaps provide strain tolerance, avoiding spalling. However, the invention in Strangman requires that the substrate surface have sufficient thickness to accommodate the grooves formed therein. For thin metal turbine shrouds with a thick ceramic coating, it becomes impractical to have a deep enough groove in the metal substrate to cause adequate shadow gaps to form in the zirconia.
Schienle et al., U.S. Pat. No. 5,352,540, entitled "Strain-Tolerant Ceramic Coated Seal", which is assigned to the assignee of this application and also incorporated by reference herein, provides a method of laser machining an array of grooves into a ceramic high temperature solid lubricant surface layer of a seal. When applied to a thin turbine shroud coated with a thick TBC layer, however, the results have not been satisfactory. Particularly with a thin substrate, the depth of the groove must be accurately controlled, so as to be deep enough to provide strain relief, but not touch the substrate. The laser machining method of Schienle does not provide the required level of control over the groove depth. Also, stabilized zirconia vapor produced by the laser machining process tends to fill in the groove behind the laser. To compensate for this back filling phenomenon, the grooves must made be excessively wide, which takes away from the sealing effectiveness of the shroud.